The present invention relates to an apparatus for warming blood and other fluids precisely to a desired temperature, prior to introduction into a patient. Banked blood and blood products are stored under refrigeration and must often be warmed before infusion. Warming is necessary so as to minimize adverse thermal reactions to the patient. Physiologic conditions such as shock, hypothermia, and cardiac disorder can be aggravated by the infusion of fluids even slightly below normal patient body temperature. Thus, a device which precisely and quickly warms stored and refrigerated fluids to body temperature for introduction to a patient is highly desirable.
Fluid warming is typically accomplished with an in-line warming device located between the blood or fluid bag and the patient's venous line. Heating occurs as the fluid passes through a heat exchanger within the warming device. Conventionally, the heat exchanger uses a disposable insert intended for single patient use only. Because the warming device is primarily used to warm blood it is often referred to as a blood warmer, yet the device is also suitable for warming IV solutions and other blood-related products intended for introduction into the patient.
There are many blood-related products available to the clinician for transfusion therapy. Whole blood, packed red cells, fresh frozen plasma and platelet concentrates are among the more commonly used products. The present invention is suitable for warming all of these fluids, as well as blood and IV solutions, too.
The American Association of Blood Banks (AABB) has set standards for the storage of blood-related products. A temperature of between about 1.degree. to 6.degree. C. is required. The AABB further recommends warming before infusion of the products into the patient and advises that the warmed blood-based products should not exceed 42.degree. C. Warming above this upper limit can cause irreparable damage to the red blood cells, including hemolysis, and possible harm to the patient.
As medical science has advanced, the demands for infusion of body fluids, particularly whole blood, have grown. Today, infusion rates range from one to two units of whole blood infused over a two hour period for a typical adult having undergone coronary surgery, to over a unit per minute in the case of a multiple wound gunshot victim being treated in a trauma center. The flow rates involved range from about 10 cc/min to 500 cc/min.
Whole blood is conventionally stored in 500 cc bags. Each such blood bag is referred to as a unit. The bags are hung on a pole above the blood warmer and above the patient. Gravity causes flow of the blood from the bag, through the warmer, and into the patient. To meet the demands of high blood infusion rates the bags are often placed inside pressure infusers that force the blood out of the bag at a higher flow rate than gravity alone can provide.
Since normal human body temperature is 37.5.degree. C., physicians desirably would like to warm all fluids to human body temperature prior to patient introduction. This is believed to provide optimal patient care and minimal thermal "shock" due to blood or fluid infusion. The goal of the blood warmer device is to rapidly warm fluids from the lowest storage temperature of 1.degree. C. to the desired patient introduction temperature of 37.5.degree. C., without any overheating, at all flow rates between about 10 to 300 cc/min, and to accommodate rapid changes in flow rates, all without over or under heating of the blood or fluid.
The challenge of wide dynamic temperature and fluid flow rates and the fact that the maximum allowable temperature (42.degree. C. per AABB) is close to the final desired output temperature (37.5.degree. C.) places high expectations and demands on the construction and operation of an effective blood warmer.
In U.S. Pat. No. 3,443,060, a plastic bag carrying blood for a patient is maintained at a thermostatically controlled temperature. The device employs a cylindrical heated mandrel around which a flat plastic bag is telescoped and an outer hollow cylindrical mandrel also telescoped over the bag and the inner mandrel. Longitudinal ribs formed on one of the mandrels press the bag walls together under the ribs to form a labyrinth of passageways through the bag. Inlet and outlet connections for conducting the blood supply to the bag and from the bag to the patient are provided. There is no teaching in the '060 patent for providing a sensor for the fluid within the bag, nor is there a teaching of a counter flow pathway for uniform heating of the fluid, nor any mention of a back-up system to prevent any over heating of the blood.
U.S. Pat. No. 4,019,020 relates to a controlled temperature fluid heater. The device is intended to maintain the fluid supply at optimum temperature for infusion into a patient by employing a flat plastic bag having a labyrinth of inner flow passageways. There is no teaching or suggestion of providing a sensor for the fluid within the bag, nor any teaching of a counter flow pathway for uniform heating of the fluid, nor any mention of a back-up system to prevent any overheating of the blood.
U.S. Pat. No. 3,485,245 shows a portable heater for heating fluids to the correct body temperature prior to intravenous injection. The fluid is directed in a serpentine-like path through a disposable, heat conductive pouch. The pouch is held between heating platens. Temperature sensors are located in the heating unit and an automatic control seeks to ensure that fluid is delivered at the correct temperature. This reference does not teach the use of a counter flow pathway to uniformly heat the fluid nor does it include any teaching of safety system(s) to prevent overheating of the blood.
U.S. Pat. No. 1,492,821 relates to an antifreezing device. Basically, the device is used to prevent water pipes, especially those in automobiles, from freezing. The device comprises a hinged box for surrounding the pipe. A heating electric coil, arranged in a zig zag pattern, is located within the box and, when power is supplied, heats the pipe. Clearly, this '821 patent neither teaches nor suggests the subject invention relating to rapid blood warming for patients.
U.S. Pat. No. 5,073,167 teaches a device which heats blood with microwaves. This method of energy transfer in itself makes for a drastically different piece of equipment. In contrast, the present inventors' method of over temperature protection depends on the gradual increase of a large mass transferring energy to the heat exchanger and the monitoring of the temperature of that large mass.
U.S. Pat. Nos. 5,245,693 & 5,381,510 show heaters for heating fluids prior to intravenous injection through a special cassette of rigid plastic and thin metal layers. The fluid is pressure regulated through a serpentine pathway in the disposable cassette. The cassette is held between two heating zones with multiple temperature sensors and an automatic control seeks to ensure that fluid is delivered at the set temperature. This preference does not teach the use of a counter flow pathway to uniformly heat the fluid nor does it rely on an energy reservoir.
Several disadvantages have been observed in prior art blood warmers, namely:
a) Overheating of the red blood cells, i.e. heating beyond the maximum allowable temperature. This can happen when, for example, little fluid is left in a blood bag and the flow rate is low; PA1 b) Rupture of blood cells due to mechanical trauma; PA1 c) Inadequate heating of the blood to the desired, optimal patient introduction temperature; PA1 d) Uneven heating of the blood when increased flow rates are demanded; and, PA1 e) Expensive single patient-use components.
Blood warming prior to patient infusion dates back to around 1941. Early blood warmers consisted of containers of warm water that the bags of blood were placed into for a period of time prior to infusion. This is similar to the procedure of heating frozen vegetables, contained in a plastic pouch, by immersing the same in heated water. A few decades later, more sophisticated water bath blood warmers, like the Dupaco Hemokinetherm and American McGaw N3001, were introduced. They used electrical heater elements to maintain a reservoir of water at substantially 37.degree. C. while the blood flowed through a disposable coil of tubing, immersed in the heated water.
A recent application of the water bath principle for blood warming is found in the Level 1 Technologies device, marketed as a Fluid Warmer. In this device, water is maintained at a fixed temperature near 42.degree. C. and pumped through a "counter current" heat exchanger tube. The disposable heat exchanger tube uses a double lumen, aluminum tube to allow blood to flow in one direction and water to flow in the other. The energy is transferred from the water through the aluminum sidewall to the blood, to warm the blood. The counter current flow gives this technology a higher warming capacity than the earlier water bath types but the disposable heat exchanger tube is rather expensive. The system seems bulky and messy to operate. The present invention uses electric heater elements. This eliminates the mess and other disadvantages associated with water heater-type fluid warmers.
The DATACHEM FloTem II dry heat fluid warmer uses large metal plates, heated with an electric heater, to raise the temperature of stored fluid prior to introduction into the patient. A length of tubing is laid next to the hot plates and fluid flowing through the tubing is heated via conduction. The system does not employ microprocessor temperature controls. In contrast, the present invention has a design wherein the actual temperature of the outlet fluid is monitored for precise control of the fluid temperature. The prior art device does not measure or display fluid temperature, only warming plate temperature.
Fenwal Laboratories' BW-5 is another example of a dry heat fluid and blood warmer. Like the DATACHEM device, the BW-5 unit also heats aluminum plates with electric heater elements. Plates are maintained at a desired temperature by a simple control circuit, again based on plate temperature. This unit incorporates a bag rather than heat conductive tubing as the mechanism for energy transfer. The BW-5 seeks to measure blood temperature by the use of a thermistor in the aluminum plate near the exit port of the heat exchanger bag. This thermistor is used to display blood temperature, but no control of the heater elements as a function of the sensed fluid temperature is accomplished or taught by the BW-5.
In contrast, the present invention uses the sensed fluid temperature to actually control the power provided to the heaters. The manner in which the blood temperature measurement is taken in the BW-5 device is such that a high degree of error can be expected and the measurement was thus not thought suitable for heat control purposes. A portion of this measurement error was attributable to the lack of precise registry and alignment between the bag and the temperature sensors. No mechanism for ensuring registry or alignment was provided. The present invention employs a pair of parallel guide rails secured to the sides of the disposable bag. The rails slide into and are received by spaced slots in the warming device. These rails are color coded and are of different sizes to prevent the user from inserting the bag incorrectly. Without these features, the temperature measurement of the fluid is not suitable for feedback to provide precise electrical/heating control. Another difference between the BW-5 and the present invention is that the BW-5 has a single heater whereas the present invention has two heaters. This results in more controllable and uniform heating.
Another device, the Animec AM-4 (made in Japan) is almost identical in design to the BW-5. It also incorporates a bag type heat exchanger and an output temperature sensor. No use is made of the output temperature for electrical power/heating control because of the same limitations on measurement accuracy believed to be found in the BW-5.
Yet another prior art dry warmer is American Medical Systems' DW1000D. Like the others, this device incorporates a solid aluminum heater and measures and displays only heater temperature. The heater in that device is cylindrical. The heat transfer bag is wrapped around the heater and is held in place by a curved plexiglass cover. While interesting in that the cover lets the operator view the fluid as it flows through the warmer, having a heater element contact only one side of the heat exchanger bag dramatically reduces the heating capacity and uniformity of fluid heating. A major disadvantage of all the "dry warmers" is that the installation of the disposable heat exchanger bag takes more time and is more difficult. Also, the extra moving parts could cause leaks in the disposables and provide for more failure modes in and of themselves. The present invention is believed far more efficient and accomplishes its task in a far more accurate and uniform manner.
Neither the DW1000D nor any of the other prior devices teach the use of slots on the warmer device and corresponding guide rails on the fluid bag which cooperate for aligning and registering the heat transfer bag into the warmer. The DW1000D has a plexiglass cover that opens up. The FloTem II and BW-5 incorporate a metal door that is opened to install the bag. Without the use of the guide rails incorporated in the present invention, a bag does not have enough rigidity to be inserted into a slot without "bunching up". With the guide rails, installation is quick and easy. The guide rails also facilitate accurate temperature measurement of the fluid and ensures more efficient and uniform heating. The guide rails ensure proper alignment of fluid input and output with the warmer.